Waveguide filter for microwave heating apparatus

ABSTRACT

A low pass filter is provided for rectangular waveguide of the type employed for launching electromagnetic energy in microwave ovens. The filter substantially rejects the transmission of harmonic frequencies of the fundamental generator operating frequency as well as adjacent energy oscillation modes. The structure described has a corrugated configuration with ribs defining alternate cavities and constrictions have cutoff frequency characteristics. Uniform and nonuniform spacing as well as dissimilar sections are discussed. The structure is electrically isolated from the broad waveguide walls to define two propagation paths for the electromagnetic energy. Lightweight conductive materials are suggested. The interrelationships of the alternating rib members provide a series of open and short circuit admittances which are analyzed to determine the desired electrical cutoff frequencies. Identical rib structure is defined on opposing sides of a reference plane extending along the longitudinal axis of the waveguide. Transforming means are utilized for impedance matching.

United States Patent 1 Ironfield Sept. 11, 1973 l l WAVEGUIDE FILTER FOR MICROWAVE HEATING APPARATUS Richard Ironfield, Williamsburg, Iowa [75] Inventor:

[73] Assignee: Amana Refrigeration, Inc., Amana,

Iowa

Filed: June 13, 1972 Appl. No.: 262,396

[52] U.S. CI. 219/1055, 333/73 W [51] Int. Cl. 1105b 9/06, I-IOlp 7/20 [58] Field of Search 219/1055; 333/73 R,

[5 6] References Cited UNITED STATES PATENTS 12/1968 Staats 2I9/10.55

1/1971 Bucksbaum 219/1055 7/1965 Walker et al. 333/73 W 10/1971 Newbould 333/73 W [57] ABSTRACT A low pass filter is provided for rectangular waveguide of the type employed for launching electromagnetic energy in microwave ovens. The filter substantially rejects the transmission of harmonic frequencies of the fundamental generator operating frequency as well as adjacent energy oscillation modes. The structure described has a corrugated configuration with ribs defining alternate cavities and constrictions have cutoff frequency characteristics. Uniform and nonuniform spacing as well as dissimilar sections are discussed. The structure is electrically isolated from the broad waveguide walls to define two propagation paths for the electromagnetic energy. Lightweight conductive materials are suggested. The interrelationships of the alternating rib members provide a series of open and short circuit admittances which are analyzed to determine the desired electrical cutoff frequencies. Identical rib structure is defined on opposing sides of a reference plane extending along the longitudinal axis of the waveguide. Transforming means are utilized for impedance matching.

11 Claims, 8 Drawing Figures 58 4 64 54 66 2 Q as; Y

/Z 90 v f f 'l/40 a 90 44 K A; x I 46 l l n 48 5 MAGNETRON ENERGY GENERATOR M ASSEMBLY 5 HIGH 35 VOLTAGE SUPPLY PAIENIED E I 1 I975 3.758.737

" sum 2 or 2 L ELECTRIC WALL T F/G 6 MAGNETC i1"- WALL I WAVEGUIDE FILTER FOR MICROWAVE HEATING APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a rectangular waveguide filter for microwave electromagnetic energy.

2. Description of the Prior Art Microwave heating includes the generation of elec tromagnetic energy by such popular sources as the magnetron oscillator used in World War II radar applications. Such generators provide a source of microwave energy which is fed to the interior of an enclosure through hollow-pipe waveguide. The oven supports a plurality of energy modes. Magnetrons are operated by domestic low frequency AC line voltages energizing power supplies capable of generating very high DC voltages. An interaction region is defined between a central emissive cathode and circumferentially disposed anode cavity resonators. The electrons are accelerated toward the cavity resonators and a spoke-like electrical space charge rotates in a substantially helical path in the interaction region to initiate the high frequency oscillations. In such devices the preferred operating TE mode is referred to as the rr-mode and, to assure the proper phasing of the energy, the mode of operation most frequently utilized is the N/2 mode where N indicates the number of cavity resonators.

In the microwave heating apparatus art the escape of high frequency energy from the oven cavity is necessarily controlled in order to comply with standards established by Federal and State regulatory agencies, suchas the Department of Health, Education and Welfare, Federal Communications Commission and the United States of America Standards Institute. The assigned frequencies for microwave heating apparatus is either 915 or 2,450 MHz. For domestic use the latter frequency is most frequently utilized. The term microwave as used in this specification is intended to refer to that portion of the electromagnetic energy spectrum having wavelengths in the order of 1 meter to one millimeter and frequencies in excess of 300MHz.

One of the problems encountered in the efficient pro tection of microwave apparatus by appropriate elongated energy seals around the access opening is created by the fact that magnetron energy generators generate harmonic frequencies, as well as, the operating frequency of 2,450 MI-Iz. Additionally, such generators provide energy oscillations in adjacent operating modes such as, for example, the (N/2) 1 mode. It is necessary, therefore, for appropriate safety consider- I ations, to provide means in energy seal structures for the absorption of the harmonic frequencies of the fundamental magnetron operating frequency. Such safety measures include additional elongated energy absorbing bodies of such materials as rubber or plastic loaded with carbon derivatives or ferrite materials and the like disposed adjacent to the oven access opening to provide a second energy seal. These bodies become rapidly heated during operation which tends to shorten life and create other problems.

A conventional microwave oven apparatus launching section for coupling the energy from the magnetron generator has a rectangular configuration. Such waveguides are intrinsically high pass filters having a threshold or cutoff frequency below which the excitation fields die exponentially. The cutoff frequency depends on the geometry of the waveguide as well as the particular energy mode excited. For normal TE mode propagation the cutoff frequency is equivalent to A =2a where a" represents the wide dimension of the waveguide. The provision of filters in rectangular waveguides for, particularly, the attenuation of higher order mode and harmonic frequencies or essentially a low pass filter has, therefore, been difficult to achieve. This has led to the acceptance of the related energy which is controlled by absorption by the energy seal materials disposed adjacent to the oven access opening.

Some primary fundamental energy seals which have evolved in the art forming electrical chokes are described and discussed in US. Pat. Nos. 3,182,169 issued to Richard Ironfield, dated May 4, 1965, and US. Pat. No. 3,584,177 issued to Arnold M. Bucksbaum, dated June 8, 1971, all assigned to the assignee of the present invention. These electrical choke arrangements commonly are provided adjacent to the access opening or in the door closure means. Such choke-type energy seals, however, are dimensioned for handling primarily only escaping energy at the fundamental frequency and operating mode. The present invention, therefore, is directed to control of higher order oscillation modes and harmonic frequencies of the fundamental operating frequency, 2,450 or 915 MHz.

SUMMARY OF THE INVENTION A In accordance with the present invention a low pass waveguide filter structure is provided for an energy launching section. A series of open and short circuit admittances having predetermined electrical cutoff parameters are formed by half sections arranged symmetrically having identical image parameters. Alternate rib members define a series of constrictions and spaces to provide the required inductances and capacitances. The spacings may be uniform or nonuniform. In microwave heating applications requiring an operating frequency of, for example, 2,450 MHz the low pass filter has a pass band or transmission region below a cutoff frequency of, illustratively, 2,800 to 3,200 MHz. The stop band or rejection region extends well above the transmission frequencies or, illustratively, 3,000 to 6,000 MHz with virtually no energy propagation in this frequency range. Adjacent magnetron higher order modes such as the (M2) 1 mode generally have frequency ranges of 3,900 to 4,200 MI-lz. Second and higher harmonics of the fundamental operating frequency arise at approximately 4,900 MHz. All these energy transmission components will, therefore, be effectively rejected. Transforming end sections are utilized to provide improved matching of the filter structure to the magnetron energy generator and oven cavity.

A number of sections of varying rib height dimensions comprise another embodiment of the waveguide filter with each section having different cutoff frequency characteristics for maintaining a sharp cutoff and high attenuation in the stop band. The filtering of higher order modes and harmonic frequencies has substantially reduced or eliminated the need for providing elongated energy absorbing bodies surrounding the oven access opening in addition to the primary energy escape seals. Hence, in place of the bodies or gaskets of a rubber or plastic composition loaded with carbon derivatives or ferrite materials and the like, unloaded plastic or rubber gasket materials can be utilized with a saving in material cost.

The waveguide filter structure may be fabricated of any lightweight metallic material, such as aluminum, to evolve the overall corrugated configuration with the alternative rib member and spaces. The waveguide filter structure is suitably secured in the waveguide launching section by any means appended to the narrow rectangular waveguide walls. The waveguide filter is spaced from the broad waveguide walls to essentially define two identical parallel transmission paths each having the desired stop band characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS Details of the illustrative embodiments of the invention will be readily understood after consideration of the following description and reference to the accompanying drawings, wherein:

FIG. 1 is a perspective view, partially broken away, of an illustrative microwave oven apparatus;

FIG. 2 is a vertical cross-sectional view of the embodiment illustrated in FIG. 1;

FIG. 3 is an enlarged perspective view of the waveguide filter structure embodying the present invention;

FIG. 4 is a diagrammatic illustration of an alternative embodiment of the invention;

FIGS. 4a and 4b are partial diagrammatic views of the embodiment in FIG. 4;

FIG. 5 is a diagrammatic view of another alternative embodiment; and I FIG. 6 is an enlargement of a portion of the embodiment illustrated in FIG. 1 showing a portion of a choketype door energy seal arrangement exemplifying the prior art, taken along the line 6--6 in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, particularly FIGS. 1 and 2, the microwave oven apparatus 10 will be described. The oven comprises top and bottom conductive walls 12 as well as side walls 14 defining enclosure 16 having an access opening (not visible) closed by vmeans of door assembly 18 which may be side or bottom-hinged, as well as the sliding type actuated by handle 20. An escutcheon control panel member 22 is disposed adjacent to the door assembly and is utilized for mounting timers 24 and 26, as well as start, stop and light buttons 28, 30, and 32.

The electromagnetic energy is generated by a magnetron energy generator assembly in dicated generally by the box 34. Such devices are considered to be well known in the art and further details may be obtained by referring to the text Microwave Magnetrons", Radiation Laboratory Series, Vol. 6, by G.B. Collins, McGraw-Hill Book Company, Inc., 1948 and U.S. Pat. No. 3,531,6l3 issued Sept. 29, 1970, to GP. Domenichini et al. and assigned to the assignee of the present invention. Such devices are energized by high voltage supply circuits also considered to be within the scope of the knowledge of the prior art and have been designated by box 36. The generated energy is coupled to the oven enclosure by a probe antenna 38 housed within a dielectric dome 40. The energy is launched into a rectangular waveguide launching section 42 with the dome extending through an aperture 44 in the waveguide section. Top wall 12 of the oven enclosure supports the waveguide section 42. The section is closed at one end by a terminating wall 46 and the antenna 38 is spaced from this wall a predetermined distance to provide for optimum launching of the microwave energy. Such energy is coupled into the oven enclosure 16 through the inner open end 48 of the waveguide section. Distribution of the energy to evolve the desired heating pattern is accomplished by any of the well-known means including, for example, mode stirrer 50 having a plurality of paddle members 52 actuated by motor 54 also supported by top wall 12. The articles to be heated within the enclosure 16 are supported on a dielectric plate member 56 spanning an indentation in the bottom wall 12 to facilitate the distribution of the electromagnetic energy on all sides of the articles.

The waveguide filter structure 58 having a substantially corrugated configuration is disposed within the launching waveguide section 42 between the energy antenna 38 and the open end 48. The spacings are preferably evenly matched between the end of the waveguide filter structure 58 and the inner edge 60 of the inner end and antenna 38 to obtain efficient matching between the respective structures. The waveguide filter structure 58 is supported by the narrow walls 62 and spaced from the broad waveguide walls 64 to define essentially two equal and identical paths for the propagation of energy indicated by arrows 66 and 68.

Attention is now directed to FIGS. 3 and 4 to proceed with an explanation and analysis of the corrugated waveguide filter 58. In an article entitled Analysis of a Wide-Band Waveguide Filter by Seymour B. Cohn, Proceedings of the IRE, June 1949, pps. 651-656, the author presents a discussion and analysis of a hollow highpass rectangular waveguide filter. Normally such propagation means have an inherent characteristic cutoff frequency which for the TE mode is twice the dimension a of the broad waveguide walls. Any energy below the cutoff frequency dies exponentially and there has been little demand in the art, therefore, for any further filter structures forrectangular waveguides in view of the intrinsic propagation characteristics. The aforementioned article seeks to increase the bandwidth of such filter structures by providing a series alternating rib members and spaces defining constrictions and cavities which provide a nondissipative filter for energy propagating within its boundaries. Each section of the overall filter structure is fabricated to have a predetermined cutoff frequency and the overall admittance (re ciprocal of impedance) of the filter is related to the short and open circuit admittances of each section. All the admittances are nonnalized with respect to the characteristic admittances of the rectangular waveguide having a width a and height h. The characteristic admittance of the guide is inversely proportional to b" if width a" is held constant. The filter structure may be provided split in identical mirror image sections and the parameters of classical filter theory are utilized in the article to determine the height of the rib sections as well as constriction in between to evolve a stop band or rejection region having a guide wavelength A v In FIG. 4 a half section is designated by the numeral 70. A series of formulae and design considerations have been presented in the article Design Relations for the Wide-Band Waveguide Filter" by the same author in the Proceedings of the IRE, July 1950, pps. 799-803. In these articles the symbol 1 refers to the width of the cavities through which the energy is propagating and the symbol 1' indicates the constrictions between the cavities. The symbol b indicates the height of the terminating waveguide section through which the energy passes. One-half of each of the waveguide filter sections 70 (H2) may be reduced to a short-circuited section indicated in drawings FIG. da. Similarly, the open circuit admittances have been analyzed by the aforereferenced author by the model indicated in FIG. 411.

Now in the instance of microwave heating predetermined assigned frequencies of 2,450 MHz or 915 MHz are involved and the energy to be distributed within the oven enclosure is desirably as close to these frequencies as possible. Wide band propagation, therefore, is of little consequence and our attention is directed solely to the suppression of extraneous magnetron operating modes and harmonic frequencies of the operating frequency. In FIG. 4 the half section indicated by the solid line 72 represents the model utilized by the aforereferenced author for computing the image admittances of the overall filter structure as well as the short and open circuit admittances of each section. Dashed line 74 indicates a reference plane forming a boundary in the authors analysis of the cutoff wavelength computations for the pass and stop bands for rectangular waveguide. In the presentinvention line 74 coincides with the longitudinal axis of a waveguide section. From the aforereferenced analysis an equation has evolved to derive a value b equal to b V l- (A /k Wherein the symbol A, indicates the cutoff guide wavelength of the filter structure, x indicates the matching point wavelength where filter inpedance will match the terminating impedance and b indicates the height of the terminating guide section. Utilizing the image perameters it is possible to calculate the height of the rib member and end portions, as well as, thickness of the intervening constricted portions. The symmetrical structure is evolved by the identical model defined by boundary line 72.

In accordance with the invention instead of transmitting energy through the waveguide filter arrangement a solid or dissipative configuration has evolved. Energy transmitted along the path indicated by arrow 66 sees a half filter section 72 and an identical path indicated by arrow 68 is formed by section 72' for the remaining electromagnetic energy. The equivalent capacitances and inductances are theoretically determined and normalized to determine respective cutoff guide wavelengths for the overall filter structure and define a stop band or rejection region having a frequency higher than the waveguide cutoff frequency. Such devices are commonly referred to in the art as low pass filters and have an upper limit of the stop bands of a finite value, illustratively, six times the cutoff frequency of the filter. By appropriate transformation of theory the ribs define the restrictions and the spaces therebetween are the cavities. The electromagnetic energy instead of traversing and being propagated through the filter arrangement is transmitted around the filter with the ends thereof terminating in transforming sections 76 and 78 for more efficient matching of the impedances of the waveguide filter to the remaining structure.

As shown in FIG. 3, the waveguide filter 58 comprises a plurality of rib members 80 and 02 defining a corrugated configuration which are substantially of the same height on opposing sides of the body member as. intervening alternating spaces 86 are formed between the rib members and for the narrow transmitting band and wide stop band are generally narrower than the cavity width (1) in the previously referenced articles.

The normalized characteristic cutoff guide wavelength of the filter structure it, as well as the desired stop band minimum limit for a predetermined height b, of the rectangular waveguide launching section can be determined with a stop band or rejection region preferably extending from 3,000 to 6,000 MHZ. In an exemplary embodiment for operation in the microwave oven apparatus at 2,450 MI-Iz the filter cutoff frequency fell between 2,800 to 3,200 MHz. The rejection band of from 3,000 6,000 MHz encompasses the adjacent magnetron energy modes as well as second harmonics of the operating frequency. For optimum matching the antenna 38 was spaced from the terminating end wall 46 of the waveguide launching section a distance of approximately 1.25 inches or one-quarter wavelength. The end of the waveguide filter structure 58 or the end of the matching transformer section 76 is positioned approximately 1.4375 inches from the antenna 38. The end of matching transformer section of the waveguide filter is disposed approximately l.4375 inches from the inner edge 60 of the open end 48 of the waveguide launching section. Suitable tapped holes 88 are provided in the body member 84 to support the waveguide structure 58 by metalor nylon screws to the sidewalls 62 and. provide for clearance and centering of the ends of the rib members and 82 with relation to the top walls 64 of the waveguide structure. In FIG. 3 the rib members have been illustrated as being nonuniformly spaced which may be desirable in some embodiments while the rib members have been shown as being uniformly spaced in FIGS. 1 and 2 for other applications. The corrugated waveguide structure may be fabricated of any lightweight metal, such as aluminum, and large strips of such structures may be fabricated quite inexpensively to be cut to any desired length. The spacing of the rib members is identical on opposing sides of the body member. Any number of sections may be utilized in the waveguide filter structure and the number of such sections is not limited by the illustrations hereinbefore described.

A large number of dissimilar sections may also be utilized with each section providing a sharper characteristic cutoff frequency for different rejection regions. Such a structure may have a tapered appearance which also assists in the matching of the filter structure impedances to those of the rectangular waveguide launching section. In FIG. 5 the first rib members 92 could, illustratively, have a stop band frequency around 3,500 Mll-Iz. Subsequent rib members 94 have a cutoff frequency characteristic value around 4,500 MHz. Rib members 96 then provide a cutoff frequency around 5,500 MI-Iz. Matching transformer sections 98 are also provided. This dissimilar arrangement of a series of sections having different cutoff frequency values may enhance the removal of all the spurious energy signals outside the desired operating mode and frequency for efficient microwave heating.

A unique low pass filter arrangement has now been evolved for rectangular waveguide sections. Due to the elimination or substantial reduction in the undesirable energy oscillations certain distinct advantages are noted over prior art microwave oven apparatus particularly, those utilizing peripheral elongated high energyabsorbing gasket materials surrounding the access opening in conjunction with a primary choke-type energy seal as shown in FIG. 6. Door assembly 18 comprises a panel member 100 and ring member 102 secured together by any conventional metallurgical means to form a unitary assembly. Perforations 104 in metal panel member 100 allow for visual observation of the oven interior during cooking while preventing the escape of any energy radiated within the oven enclosure 16. An outer window member 106 is supported within ring member 102. An inner window assembly R08 with a transparent region also renders the perforations inaccessible to damage and simplifies cleaning of the oven interior. Both windows may be fabricated of a thermoplastic material. A stud 110 secured to frame member 112 provides for press fitting into apertures in panel member 100. Window member 106 may be secured in position by a suitable adhesive as well as door trim members 114 shown in FIG. 1.

A door-type electrical choke arrangement 116 extends peripherally around the door to form an elongated primary energy seal mating with slightly tapered walls 118 of the oven conductive walls 12 and 13 to assure a snug fit after closing. The choke arrangement comprises a peripheral upstanding wall section 120 which defines with the opposing tapered conductive wall 118 an elongated electromagnetic energy escape path 122 extending peripherally around the access opening. The point of origin of the energy is indicated at the gap 124. Ring member 102 has a substantial step configuration to provide a conductive wall surface 126 forming a part of the choke arrangement as well as a front lateral member 128 overlapping the peripheral walls of the oven. Ring member 102 defines with wall section 120 a second electromagnetic energy path 130 of predetermined dimensions for energy entering through the peripheral gap 124. The parallel paths 122 and 130 are filled with bodies 132 of a dielectric material, such as polystyrene or polypropylene. The entrance and exit to the frequency sensitive cavity defined by the choke walls circumscribing the path 130 is provided by gap 134. The foregoing arrangement may be considered to be the primary high frequency energy seal offering a path of least resistance for any energy at the operating frequency escaping around the peripheral gap 124.

in accordance with the principles of energy transmission, such a choke arrangement is selected primarily to provide a high series reactance at the choke opening and to reflect a short circuit from a terminating wall surface 136 to the energy. The choke dimensions are typically selected to provide a short circuit at the point of origin or gap 124 of the escaping energy or approximately one-half a wavelength of the operating frequency. The modes primarily in the operating mode are effectively attenuated by such energy seals, however,

- higher order modes as well as harmonics of the operattended periods of operation by absorption and may become warped, charred or cease to function properly.

In the practice of the present invention, the elimination or substantial reduction of the harmonic frequencies will substantially reduce or eliminate the need for the additional high energy absorbing bodies of the prior art adjacent to the door opening. In place of such peripheral gaskets, commonly of a black composition, more esthetic colors may be employed such as white or vivid colors in plastic materials which will render the oven apparatus more appealing in addition to being easy to clean. Prior art carbon loaded gasket materials are costly and may now be readily replaced by materials costing one-tenth of the prior art amount. The additional cost of the low pass waveguide structure to effectively suppress the undesired modes and harmonic frequencies involves relatively minimal expense so that overall savings of 50 percent or better of the original energy absorbing material gasket cost may be realized.

There is thus disclosed an effective low pass filter for rectangular waveguide, particularly for use in microwave oven apparatus where a narrow operating frequency band is involved. The filter structure described has a corrugated configuration. The alternate rib members defining the intervening spaces and constricted body portion may be uniformly or nonuniformly disposed in each section. Additionally, dissimilar filter sections having varying cutoff frequency characteristics may also be utilized in the practice of the invention. Other variations, modifications and alterations will be evident to those skilled in the art. It is intended, therefore, that the foregoing description of the invention and the illustrative embodiments be considered in the broadest aspects and not in a limiting sense.

I claim:

1. In combination:

an enclosure;

means for generating electromagnetic energy having at least a predetermined operating frequency; means for launching said energy within said enclosure; and

low pass filter means comprising a body member having alternately spaced rib members mounted in said launching means; said rib members and alternate spaces defining an inductance and capacitance in series along said launching means to provide a predetermined propagation characteristic to substantially prevent transmission of energy at harmonic frequencies of said operating frequency. 2. The combination according to claim 1 wherein said. rib members are spaced uniformly.

3. The combination according to claim 1 wherein said rib members are spaced nonuniformly.

4. The combination according to claim 1 wherein said filter means comprise a body member having spaced rib members extending symmetrically from a predetennined reference plane.

5. Heating apparatus comprising: an enclosure defined by conductive walls; an electromagnetic energy generator having a fundamental operating frequency characteristic;

rectangular waveguide having broad and narrow walls for coupling said energy from said generator to said enclosure;

low pass filter means comprising a body member having alternately spaced rib members mounted in said waveguide;

said rib members and alternate spaces defining an inductance and capacitance in series along said waveguide to provide a propagation characteristic to substantially prevent transmission of energy at harmonic frequencies of said operating frequency.

6. The apparatus according to claim 6 wherein said rib members extending transversely to the direction of energy propagating along said waveguide.

.7. The apparatus according to claim wherein said rib members extend parallel to the broad waveguide walls and are spaced therefrom to define a propagation path.

8. The apparatus according to claim 6 wherein said characteristics.

, III It UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,758,737 Dated September 11, 1973 Invent r( Richard Ironfield It is certified that error aopears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Claims Column 9, line 9, change "6" to 5 Column 9, line 16 change "6" to 5 Column l0, line 3, Change "6 to 5 Column 10, line 7, change "6" to 5- Signed and sealed this 19th day of Msrch 1 974.

(SEAL) Attest;

EDWARD M.FLETCHER,JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents F ORM PO-1050 (10-69) USCOMM-DC 60376-P69 k us. GOVERNMENT PRINTING QFFICE I969 o--ass-aa4 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION I Patent No. 3,758,737 i o n Dated September 11, 1973 Inven fl Richard I'ronfiel'd It is certified that error appears in the above-identified patent and that said Letters Patent are heteby corrected as shown below:

In t'ne Claims Column 9, line 9, change "6 to 5 Column 9, line change "6" to 5 Column 10, line 3, change ',"6" to 5.

Column 10, line'7, change "6? to 4 5 Signed and sealed this i9th day of March 1974.

(SEAL) Attest:

EDWARD M.FLETCHER,JR.R c. MARSHALL DANN Attesting Officer Commissioner of Patents FORM PO-1050 (10-69) T USCOMWDC i U.S. GOVERNMENT PRINTING QFFICE "ll 0-388-334 

1. In combination: an enclosure; means for generating electromagnetic energy having at least a predetermined operating frequency; means for launching said energy within said enclosure; and low pass filter means comprising a body member having alternately spaced rib members mounted in said launching means; said rib members and alternate spaces defining an inductance and capacitance in series along said launching means to provide a predetermined propagation characteristic to substantially prevent transmission of energy at harmonic frequencies of said operating frequency.
 2. The combination according to claim 1 wherein said rib members are spaced uniformly.
 3. The combination according to claim 1 wherein said rib members are spaced nonuniformly.
 4. The combination according to claim 1 wherein said filter means comprise a body member having spaced rib members extending symmetrically from a predetermined reference plane.
 5. Heating apparatus comprising: an enclosure defined by conductive walls; an electromagnetic energy generator having a fundamental operating frequency characteristic; rectangular waveguide having broad and narrow walls for coupling said energy from said generator to said enclosure; low pass filter means comprising a body member having alternately spaced rib members mounted in said waveguide; said rib members and alternate spaces defining an inductance and capacitance in series along said waveguide to provide a propagation characteristic to substantially prevent transmission of energy at harmonic frequencies of said operating frequency.
 6. The apparatus according to claim 6 wherein said rib members extending transversely to the direction of energy propagating along said waveguide.
 7. The apparatus according to claim 5 wherein said rib members extend parallel to the broad waveguide walls and are spaced therefrom to define a propagation path.
 8. The apparatus according to claim 6 wherein said body member is supported by said narrow waveguide walls.
 9. The apparatus according to claim 6 wherein said filter means comprise a body member having spaced rib members symmetrically disposed from opposite sides.
 10. The apparatus according to claim 6 and means for matching the impedance electrical characteristics of said filter means and said waveguide means disposed at the ends of said body member.
 11. The apparatus according to claim 5 wherein the outer extremities of said rib members are spaced from opposing broad walls to define parallel energy propagation paths having substantially identical transmission characteristics. 